The most common and most likely answer to this crossword clue is the 8 letter word UNSTRUNG
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Buddy explains!

The straight part
We know that "Like a useless guitar" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case like a useless guitar = unstrung.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does UNSTRUNG mean?
UNSTRUNG verb- To remove the string or strings from.
- To shake the nerves of; to cause anxiety or panic in.
- To defuse or relax.
- To remove from a string; to release something that has been strung up or strung together.
- To untie.
- (COBOL) To split (a text string) into smaller strings by separating on a delimiter.
- Not strung